The present invention relates to a paper feeder for feeding sheets of paper by separating them one by one, by means of a paper feed roller and a separation pad pressed thereto, and, more particularly, to an image forming apparatus equipped therewith.
An apparatus for feeding paper in a printer and a copying machine typically is provided with a device to separate each of the sheets of paper loaded on a paper cassette or a manual feed tray and to feed them into the apparatus. FIG. 8 shows an example of such a paper feeder. As shown in FIG. 8, the leading edge of paper 16 is pressed against a paper feed roller 200 by a separation pad 201. Sheets of paper 16 are fed, after being separated from one another, using the difference among the friction coefficient μPM of the separation pad 201 with respect to the paper 16, the friction coefficient μMM between the sheets of paper 16, and the friction coefficient μRM on the outer peripheral surface of the paper feed roller 200 with respect to the paper 16. To put it another way, using a paper feed roller 200 and a separation pad 201, where μRM>μPM>μMM, when the paper feed roller 200 is rotated to feed the paper 16, sheets of paper 16 are separated and are fed.
To ensure smooth feed of the paper 16 in the direction of paper feed, the surface of the separation pad 201 must be pressed against the outer peripheral surface of the paper feed roller 200 with a uniform pressure. If it is pressed with an uneven pressure, the paper may be fed in a slanting direction close to the separation pad, and a paper jam may occur.
To press the surface of the separation pad 201 against the outer peripheral surface of the paper feed roller 200 with a uniform pressure, the separation pad 201 is put in a position in which it is freely rotatable about the shaft 204 provided on the separation holder 202, so that the separation pad 201 is located vertical to the tangential force L of the outer peripheral surface of the paper feed roller 200 and parallel to the feed direction of the paper 16, as shown in FIG. 9, for example. More specifically, a U-shaped groove 203 is provided on a flange extending from the bottom of the separation pad 201, and the separation pad 201 is mounted by way of this flange on the outer peripheral surface of the shaft 204. The size of the groove 203 is greater than the outer diameter of the shaft 204 to ensure that the separation pad 201 can be rotated smoothly in conformity with the profile of the outer peripheral surface of the paper feed roller 200 or the surface profile of the paper 16.
In this connection, the friction coefficient μRM on the outer peripheral surface of the paper feed roller 200 with respect to the paper 16 is different from the friction coefficient μPM on the surface of the separation pad 201, as described above. Since the size of the U-shaped groove 203 is greater than the outer diameter of the shaft 204, as described above, there is a slight play between the separation pad 201 and the separation holder 202. Thus, when paper 16 is fed between the separation pad 201 and the paper feed roller 200, the paper 16 rubs against the separation pad 201 and a stick-slip condition occurs. Noise may be caused by the vibration produced at this time. This problem is likely to occur especially when there is a greater surface friction coefficient, as in the case of an OHP (overhead transparent film), or when there is a great displacement volume between the separation pad 201 and the paper feed roller 200, as in the case of cardboard, accompanied with an increase in the pressure of the separation pad 201 and in the friction force between the separation pad 201 and the paper 16. Further, a stick-slip condition is more likely to take place with an increasing feed rate of the paper 16 by the paper feed roller 200, with the result that abnormal noise will be produced.
To prevent abnormal noise from occurring at the time of paper feed, the following technique is proposed:
(1) Modify the weight and shape of the separation pad 201 and the separation holder 202 to shift the resonant frequency between the separation pad 201 and the separation holder 202.
(2) Reduce the pressure of the separation pad 201 that is applied to the paper feed roller 200, thereby minimizing the possibility of a stick-slip condition being generated.
(3) Press the separation pad 201 against another member, for example, the frame of the printer, with the paper feeder mounted thereon, thereby reducing the vibration of the separation holder 202 (e.g. see Japanese Application Patent Laid-open Publication No. 2000-191165 (pages 3 and 4, FIGS. 1 and 3)).
However, the method (1) is incapable of meeting the frequency of the abnormal noise caused by differences in the thickness or material quality of the paper 16. Abnormal noise may occur, depending on the type of paper 16.
The method (2) causes the separation capability of the paper 16 to deteriorate, with the result that a plurality of sheets of paper 16 may be fed at one time.
The method (3) fails to reduce the vibration of the separation pad 201. The frame is hit by the separation holder 202 through the vibration transmitted from the separation pad 201, and abnormal noise is produced as a result. Alternatively, it is also possible that the frame, or another portion in contact with the frame, is vibrated by the vibration transmitted from the separation pad 201, and abnormal noise is generated. Further, a failure other than abnormal noise occurs in the following case: When an apparatus equipped with this paper feeder is a laser printer, the printed image may be disturbed if vibration is transmitted to the optical unit of the apparatus.